This invention relates to a composite zinc- or zinc alloy-plated metal sheet which has good press formability and good bare (as-plated) and post-painting corrosion resistance as well as a remarkably improved spot; weldability and which can be improved in low-temperature impact resistance. This invention also relates to a method for the production of such a plated metal sheet. The composite zinc- or zinc alloy-plated metal sheet is suitable for use in automobiles, electric appliances, and building materials.
Surface-treated steel sheets are used in various applications including automobiles, electric appliances, and building materials. In particular, the use of rust-preventing steel sheets having a rust preventive surface coating formed thereon has been promoted in automobiles, particularly in automobile panels, to be sold in cold regions, in which they are exposed to a severe corrosive environment due to the use of rock salt on roads in order to prevent freezing. As rust-preventing steel sheets are used in an increased quantity, requirements therefor with respect to rust preventing properties have become increasingly strict. For example, such steel sheets are required to have a long-term corrosion resistance intended to resist perforative corrosion for 10 years and surface rusting for 5 years.
In addition to corrosion resistance, rust-preventing steel sheets are required to have press formability capable of withstanding severe press forming such as deep drawing, and spot weldability enabling bonding by resistance spot welding, which is widely employed, particularly by continuous spot welding with an increased number of weld spots. They are also desired to have post-painting corrosion resistance, i.e., the capability of forming a painted film having good adhesion in a painting step subsequent to a press forming step such that they exhibit corrosion resistance in areas where the painted film is injured and in edge faces. Furthermore, it is desired that rust-preventing steel sheets for use in outer panels of automobiles to be sold in cold regions have cold temperature impact resistance good enough to prevent the adhesion of the rust-preventive surface coating from being damaged by an impact, e.g., caused by hitting a pebble thereon in a low-temperature region (-50.degree. C. to 0.degree. C.).
The most popular rust-preventing steel sheets are zinc-plated steel sheets prepared by electroplating or hot dipping (galvanizing) with zinc metal. However, zinc-plated steel sheets do not have an adequate long-term corrosion resistance unless the zinc coating has a large thickness. Such a thick zinc coating not only tends to cause seizure with a die in press forming, thereby adversely affecting the press formability, but also results in deterioration in spot weldability due to accelerated damage of electrodes In a spot welder caused by diffusion of zinc into the electrodes.
In order to eliminate one or more of these disadvantages, a variety of zinc alloy-plated steel sheets have been proposed. These have a coating of a zinc alloy with one or more metals such as Fe, Co, Mn, Cr, Al, or the like. Some of these sheets such as those having a Zn-Ni or Zn-Fe alloy coating, have already been used in practical applications.
Zinc alloy-plated steel sheets generally have improved as-plated corrosion resistance to perforation and can protect the substrate steel sheet with a relatively thin coating. However, their press formability and spot weldability are still less than satisfactory. Furthermore, some zinc alloy coatings tend to be readily stripped off due to powdering during press forming, thereby deteriorating the press formability and corrosion resistance. Zinc alloy-plated steel sheets also suffer from an unstable post-painting corrosion resistance and a deteriorated low-temperature impact resistance.
In the following description, the term "zinc-base plating" is used to include both zinc plating and zinc alloy plating. Similarly, the term "zinc-base plated" steel sheet or metal sheet encompasses both a steel or metal sheet having a zinc plated coating or a zinc alloy plated coating.
For further improvement of zinc-base plated steel sheets, multilayer zinc-base plated steel sheets which have a lower zinc-base plated layer and one or more upper coating layers have been proposed.
Japanese Patent Application Laid-Open (Kokai) No. 60-215789 (1985) describes a duplex plated steel sheet having a lower zinc plated coating at a weight of 10-300 g/m.sup.2 and an upper zinc alloy plated coating at a weight of 1-20 g/m.sup.2 which contains one or both of Ni and Co in a total amount of 15-30 wt %.
Japanese Patent Publication (Kokoku) No. 58-15554(1983) describes a duplex plated steel sheet having good applicability to chemical conversion treatment and electrodeposition, thereby exhibiting good post-painting corrosion resistance. The duplex plated steel sheet comprises an alloyed galvanized steel sheet having a thin Fe-based coating formed thereon by flash plating.
Japanese Patent Application Laid-Open (Kokai) No. 1-172578(1989) describes a duplex plated steel sheet comprising an alloyed galvanized steel sheet having thereon a layer of an anhydrous alkali metal salt of an oxide of a metalloid such as boron or phosphorus.
Even in these multilayer zinc-base plated steel sheets, the corrosion resistance basically depends on the lower zinc-base plated coating. Therefore, in order to achieve good corrosion resistance desired by users, the lower zinc-base plated layer must be a thick coating, thereby adversely affecting the press formability and spot weldability.
It is known that a composite organic-inorganic coated steel sheet comprising a zinc-base plated steel sheet having thereon a chromate film and a thin organic resin coating has a remarkably improved corrosion resistance due to an anticorrosive effect of the chromate film and a function of the resin coating as a barrier shielding from a corrosive environment. However, such a composite coated steel sheet does not have a satisfactory post-painting corrosion resistance since the barrier function of the resin coating is lost in injured areas and edge faces.
Accordingly, there remains a need for a surface treated steel sheet having various properties required for automobile panels, i.e., press formability and spot weldability good enough for assembly while maintaining high corrosion resistance, as well as good low-temperature impact resistance and applicability to painting so as to give improved post-painting corrosion resistance.